The present invention relates to a multilayered highly transparent biaxially oriented polypropylene film having excellent twist properties, which is suited, in particular, for twist wrapping. The invention is also directed to a process for producing such films and to their use.
Polypropylene (PP) films distinguished by good twist properties are known. For example, GB-A-1,231,861 discloses a biaxially oriented polypropylene (boPP) film which is produced by means of the bubble process and which can be readily twisted. Twistability is imparted to the film by adding to the PP homopolymer a low molecular weight hydrocarbon resin and by orientating the film in the machine direction. To facilitate processing on high-speed wrapping machines, the film additionally comprises an antistatic agent. Due to its principal orientation in the longitudinal direction, the film tends to split during the wrapping, for example, of candies, i.e., the twirled end may tear off at the twisting point. Furthermore, the film has unsatisfactory running properties on high-speed wrapping machines, and the optical properties and the shrink of the film are not completely satisfactory.
Moreover, the process for producing a film having a predominant orientation in the longitudinal direction, as described in the above publication, is very complicated, requiring balanced orientation by means of bubble process and subsequent stretching/shrinking in an off-line process. This makes the film relatively expensive.
DE-A-35 35 472 also relates to a film which is well suited for twist wrapping. By the addition of siloxanes and anti-blocking agents to the top layers, the desired favorable processing properties on automated high-speed twist-wrapping machines are imparted to the film. A disadvantage of these films is that relatively high resin contents are required for good twistability. Exemplary contents of 25% are mentioned. This drastically increases the price of the film. If the resin content of the above film is reduced, the required twisting properties are not achieved. Moreover, the film does not, in all cases, meet the desired standards with respect to shrinkage and optical properties.
EP-A-317,276 relates to a film for twist wrapping. Due to the stretching parameters indicated (.lambda..sub.1 =3.5, .lambda..sub.t =9-10), the film is more highly oriented in the transverse direction. Films of this type leave much to be desired with respect to their twisting properties.